The present invention relates to a process and an apparatus for coating a substrate with a film which contains vaporizable solvent components and solid components, by means of an electric discharge, and for drying the film on the substrate.
It is known to produce printed circuit boards in general by dry-film laminating or by wet coating with a liquid resist. Examples of conventional resist coating methods are dip coating, rolling, casting, screen printing and, in particular for the coating of thin laminates, electrostatic spraying processes.
The principle of the electrostatic spraying process is to disperse a liquid resist via a spraying member into superfine drops, then charge the drops electrically by high voltage and deposit them on a grounded workpiece, for example a substrate. On the surface of the substrate, the drops run to form a wet film, which is subsequently dried.
European Patent 0,195,041 discloses an electrostatic spraying process for printed circuit board coating, in which the substrate is moved substantially perpendicularly to the direction of the spraying apparatus in an electrostatic field, which is generated by a high-voltage generator. A liquid resist is sprayed in the direction of the substrate, which is carried by a conveyor belt, which crosses through an electrostatic field. An electrode of the electrostatic apparatus is connected to ground, while the spraying apparatus for the liquid resist is connected to the high-voltage electrode of a power generator. The spraying apparatus comprises an air turbine, a spraying bell specially developed for liquid resist and means for supplying the liquid resist. The spraying bell is charged, for example, with negative DC voltage in the range from 40 to 90 kV.
Various drying processes and apparatus are used in the drying of large-area substrates in web form to which liquid films have been applied. The applied liquid films generally consist of vaporizable solvent components, which are removed from the liquid film during the drying process, and of non-vaporizable components, which remain on the substrate material after drying.
The coating of metal webs with light-sensitive films which are then made up into printing plates requires special installations in order to ensure the desired product quality of the liquid resist films.
DE-Al 37 05 482 also discloses a process and an arrangement for the surface pretreatment of plastic by means of an electric corona discharge. The arrangement comprises a generator, which applies a high-frequency alternating current of high voltage to the discharge electrode of the corona discharging means, the metal core of a cylinder serving as grounded counter electrode, over the circumferential coating of which cylinder a film web is transported. A housing surrounds the discharge electrodes of the corona discharging means and is connected via a line to an atomizing means, in which a liquid is atomized into drops of an aerosol, capable of suspension, by means of a piezoelectrically-operating ultrasonic vibration system or by two-component atomizer nozzles operating at ultrasonic velocity. A fan, the flow rate of which can be regulated, is connected to the atomizing means and conveys the carrier gas, for example air, for the aerosol through the atomizing means into the corona discharging means.